Mirrors or optical apparatuses to reflect, focus or concentrate impinging electromagnetic radiation are well known in the art. At least three of the properties that a mirror must have are that the optical surface must have an accurate contour, that the optical surface be specular (smooth or polished) and that the optical surface reflects the appropriate electromagnetic radiation. Although mirrors have been manufactured for use in terrestrial applications as well as outer space applications, for extra-terrestrial application, the mirror must withstand other environments such as UV radiation, temperature cycling, temperature gradient, contamination, self contamination, and the impact of particles (such as atomic particles, and space debris) impinging on the optical surface of the mirror. Currently, the substrate of mirrors that support the optical surface have been made from material such as glass, metal or ceramic, all of which require certain thickness and mass for surface contour accuracy and stability. Graphite Fiber Reinforced Composite (GFRC) materials are also well known in the art. GFRC structures can have high stiffness and stability at extremely low mass. However, fibers from a GFRC structure can have a surface morphology that impacts the optical surface.
In addition, for extra-terrestrial applications the mirror must be thin, less than 0.35 mm in thickness, and light weight, less than 0.7 kgm/sq. meter mass/area, and be able to survive the temperature range of −180 C to +150 C, including multiple cycles between these extremes, without incurring permanent distortion of degradation of the mirror surface.
Prior art methods to provide an optical surface to a GFRC substrate have involved techniques such as direct replication. In direct replication, a thick layer of resin such as epoxy is applied to a contoured GFRC substrate, and then cured against a mandrel having a polished surface to create a smooth specular surface. The thickness of the resin layer depends on the level of local roughness and large scale contour errors, which results in differences between the substrate front surface and the mandrel which the replication resin fills in. Further the front surface of the specular resin layer has a thin film vacuum deposited metallic coating to provide a high spectral reflectance to the specular surface. The metallic coating can be applied in one of two ways. First, the reflective coating can be applied to the mandrel prior to replication of the resin layer, in which case the reflector coating is released from the mandrel already bonded to the resin. Alternatively the reflector coating can be applied to the replicated resin surface after the resin layer is cured by conventional optical thin film vacuum deposition coating methods, such as ion beam sputtering or the like. The disadvantage of using the direct replication method is that the resin has to be relatively thick, on the order of 0.5 mm or more, and can crack and degrade when exposed to thermal and humidity cycling across the temperature and humidity ranges of terrestrial and aerospace applications. These effects are exacerbated by the generally high Coefficient of Thermal Expansion (CTE) and coefficient of moisture expansion (CME) of the resin, compared to the CTE and CME of the GFRC substrate. Another disadvantage of the direct replication approach is the need to get good, bubble-free contact between the resin and the replication mandrel, and the need to apply a release layer to allow the resin on the GFRC substrate to be released from the mandrel. Referring to FIG. 1 there is shown a cross-sectional view (highly exaggerated) of the mirror of the prior art made by the direct replication method.
Other methods for providing a specularizing layer for the mirror surface include adding a polishable metal or amorphous (glassy) surface layer or a thick plastic layer directly to the GFRC substrate, and subsequently grinding and polishing this layer using conventional optical manufacturing methods. The disadvantage of this method is that the added layer of metal or plastic has to cover irregularities of varying thickness adding mass, thermal instability, and the risk that surface grinding and polishing can punch through or perforate the specularizing layer, exposing the substrate. The front layer, made from a dense polishable material, can also introduce local or global contour errors into a thin substrate due to CTE mismatch stresses.
Accordingly, it is one object of the present invention to provide a mirror that uses a thin substrate and is therefore, light weight and can be manufactured to withstand the hazardous environment of outer space.